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The potassium ethyl sulphate is filtered and washed with a
little spirit or methylated spirit.1
Crystallisation.—The substance should now be recrystal-
lised. The success of many operations in practical organic
chemistry depends upon skill in crystallisation. The first essen-
tial is to select a suitable solvent, that is, one which dissolves
much more of the substance at a high than at a low temperature.
To discover a suitable solvent a small quantity of the substance
(o* i gram is sufficient) is placed in a test-tube and a few drops
of the solvent poured in. The common solvents are water,
methyl and ethyl alcohol, ethyl acetate, acetic acid, acetone, benz-
ene (also toluene and xylene) nitrobenzene, petroleum spirit and
ligroin, chloroform and carbon tetrachloride. If the substance
dissolves on shaking without warming or does not visibly
diminish on boiling, it may be discarded as unsuitable. If it
dissolves on heating or boiling and crystallises on cooling in
considerable quantity, it may be employed. Sometimes solutions
can be supercooled. In such cases, rubbing the sides of the
test-tube with a glass rod will cause the substance to deposit. A
convenient method of crystallisation may be occasionally em-
ployed by using two miscible solvents in one of which the
substance is soluble and in the other insoluble. The substance
is then dissolved in a small quantity of the first solvent and
the second added gradually until a turbidity appears. Alcohol
and water, and benzene and petroleum spirit are often used in
conjunction in this way. If a substance of low melting-point is
to be crystallised care should be taken that sufficient solvent
is present to prevent the substance separating at a temperature
at which it is still liquid. The interval of temperature may be
increased after the solution has reached the ordinary tempera-
ture, by cooling it in a freezing mixture, when some of the
solid will be deposited.
In the present instance spirit or methylated spirit (purified)
will be found an efficient solvent for potassium ethyl sulphate.
The following is the mode of procedure when a volatile or in-
flammable solvent is used : the substance is placed in a round
flask attached to an upright condenser and heated on the water-
bath. The form of apparatus is that already described (see Fig.
1 If methylated spirit is used it must be purified according to the method described
on p. 48